There has been a rapid advance of research and development recently, for a high density optical disc system capable of conducting recording and reproducing of information by using a violet semiconductor laser having a wavelength of about 400 nm. As an example, on an optical disc conducting recording and reproducing of information under the specifications including NA 0.85 and light source wavelength 405 nm, namely, on the so-called Blu-ray Disc (BD), there can be recorded information of 20-30 GB per one side of the optical disc having a diameter of 12 cm that is the same in terms of size as DVD (whose specifications include NA 0.6, light source wavelength 650 nm and storage capacity 4.7 GB), and on an optical disc conducting recording and reproducing of information under the specifications including NA 0.65 and light source wavelength 405 nm, namely, on the so-called HD DVD, there can be recorded information of 15-20 GB per one side of the optical disc having a diameter of 12 cm. Hereafter, the optical disc of this kind is called “high density DVD” in the present specification.
Incidentally, if an optical pickup apparatus can only conduct recording and reproducing of information properly for the high density DVD of this kind, the commercial value of the optical pickup apparatus is not sufficient. When giving consideration to current realities where DVD and CD on which various pieces of information are recorded are on the market, it is not sufficient to be capable of conducting recording and reproducing of information properly for high density DVD only, and being capable of conducting recording and reproducing of information properly also for conventional DVD and CD owned by a user, for example, leads to enhancement of commercial value as an optical pickup apparatus of a compatibility type. From this background, an optical system used for the optical pickup apparatus of a compatibility type is required to obtain an excellent spot, for conducting recording and reproducing of information properly for any of high density DVD, and conventional DVD and CD. Though the optical pickup apparatus capable of conducting recording and/or reproducing of information on a compatibility basis for DVD and CD has been put to practical use, the apparatus is still demanded to be smaller in size, thinner in thickness and lower in cost.
When trying to conduct recording and/or reproducing of information for optical discs in different kinds in this case, there exists a problem that the aperture restriction corresponding to numerical aperture NA in the case of using each optical disc is needed. However, when trying to conduct the aperture restriction by using a mechanical shutter that makes an aperture diameter to be variable, the structure becomes complicated and cost increase is caused. For this problem, Patent Document 1 discloses a technology to place a glass flat plate covered by a wavelength selective film in front of an objective lens, and thereby to make the objective lens to have a function to regulate to the aperture corresponding to the wavelength of the light flux to be transmitted. Further, each of Patent Documents 2, 3 and 4 discloses a technology to cover directly the optical surface of the objective lens with a wavelength selective film, and thereby to give a function of aperture restriction to the objective lens.
(Patent Document 1) TOKKAIHEI No. 11-161997
(Patent Document 2) TOKKAI No. 2001-264533
(Patent Document 3) TOKKAI No. 2000-20998
(Patent Document 4) TOKKAI No. 2001-330770
However, in each of Patent Documents 1, 2 and 3, a function of aperture restriction is given selectively, concerning light fluxes respectively for DVD and CD in terms of wavelength. That is, there is neither disclosure nor suggestion, concerning the selective aperture restriction for a light flux with short wavelength of about 400 nm and for a light flux with wavelength other than the aforementioned wavelength. Further, the film described in Patent Document 4 does not transmit the light flux with short wavelength of about 400 nm, and it is difficult to use it for the aperture restriction concerning the light flux with short wavelength of about 400 nm.